This invention concerns a method for manufacturing heat-stable polyesters with decreased terminal carboxyl groups, particularly by using phosphoric acid compound with cyclic carbonates and a catalyst.
It is well known that removing terminal carboxyl groups (designated [COOH] below) is an effective way of increasing the heat stability, that is, the resistance to hydrolysis and amine decomposition, of polyesters. Various methods for decreasing the [COOH] of polyesters have been proposed, one of which is a method of reacting a polyester and a cyclic carbonate compound. It is known that in this method it is advantageous to use an alkali halide compound as a catalyst. However, the present inventors have investigated this method, and judged that there is a problem with it. The [COOH] groups of the polyesters which are decreased by the reaction with the cyclic carbonate compound increase again at subsequent stages, such as those of polycondensation, melting and molding, etc.